270 Mr Buchanan's Report on the Bridge of Suspension 
quence of this, tend to draw the sockets together, this upper 
b&r resists that thrust, and keeps the sockets at their proper dis- 
tance. It is along this upper bar that the iron-plates for carry- 
ing the materials of the roadway are laid and made fast ; and, 
; n order to sustain it in the middle, and throw the whole of the 
weight on the under arch, a series of bearers are raised on the 
tinder bar, up to the level of the upper one, and at every 2\ 
feet in its length. These being properly fastened at top and 
bottom, serve to unite the upper and under bars, and to form a 
complete frame, extending from side to side of the bridge, trus- 
sed in every part, and capable of withstanding all the weight 
which can ever be laid upon it. — [Here follow various additional 
particulars regarding the construction and dimensions of the dif- 
ferent parts of the roadway, after which description the author 
proceeds] : 
When all these arrangements are duly carried into effect, we 
shall obtain a connected mass of strength and stiffness as well 
as elasticity, sufficient to withstand any pressure or motion to 
which it will ever be subjected. For supposing a vessel were 
to strike the bridge, of 200 tons burden, and driving at the rate 
of six miles an hour, it may be shewn by calculation, that its 
momentum would not have the effect of pushing forward an 
equal weight, farther than the space of a foot. But the strength 
and flexibility of the roadway are greatly superior to this. It 
Would move or bend laterally at least three or four feet without 
injury ; and it would require a weight of at least six times that 
of the vessel, to produce such a deflection. Besides the roadway^ 
We have the chains, which, if the roadway were yielding, would 
resist of themselves many times the momentum of the vessel; 
And, as a farther security, if it were thought necessary, it 
would be easy to extend between the opposite pillars, and on a 
line with the roadway, a protecting cable, to check the motion 
of the vessel before it reached the bridge. 
The roadway is connected to the main-chains by the suspends 
ing rods. These vary in their length according to their situa- 
tion in the bridge, being longest at the pillars, and diminishing 
to nothing at the centre. They are attached below in the man- 
ner already described : two rods three-fourths of an inch in dia- 
meter being placed under the chains at the distance of 12 inches 
